1.8 Recall that atoms consist of a central nucleus, composed of protons and neutrons, surrounded by electrons, orbiting in shells 1.9 Recall the relative mass and relative charge of a proton, neutron and electron 1.10 Understand the terms atomic number, mass number, isotopes and relative atomic mass 1.11 Calculate the ralative atomic mass of an element from the relative abundances of its isotopes 1.12 Understand that the Periodic Table is an arrangement of elements in order of atomic number 1.13 Deduce the electronic configurations of the first twenty elements from their positions in the Periodic Table 1.14 Deduce the number of outer electrons in a main group element from its position in the Periodic Table
Subatomic Particles
Particle
Relative mass
Relative charge
Where in atom?
Protons +
1
+1
Nucleus
Neutrons 0
1
0
Nucleus
Electrons -
(1/2000) or just 0
-1
Shells
Relative=as compared to...
The nucleus of each atom specie can be described by just two numbers:
The atomic number, Z, which is the number of protons in the nucleus of an atom;
The mass number, A, which is the number of protons and neutrons in the nucleus of an atom
All this information about each atom can easily be shown in a short hand notation like in the example below which shows the short hand notation of a Lithium atom;
The proton number is also equal to the charge on the nucleus and, as an atom is a neutral particle, to the number of electrons orbiting around the nucleus.
Using the short hand notation or your periodic table you should be able to find the number of protons, neutrons and electrons in each atom. Remember that in each atom:
The number of protons=The number of electrons
The number of neutrons=The mass number - the number of protons
Element
Symbol
Atomic Number
Number of Protons
Number of Electrons
Mass Number
Number of Neutrons
Hydrogen
H
1
1
1
1
0
Helium
He
2
2
2
4
2
Carbon
C
6
6
6
12
6
Nitrogen
N
7
7
7
14
7
Oxygen
O
8
8
8
16
8
Neon
Ne
10
10
10
20
10
Sodium
Na
11
11
11
23
12
Magnesium
Ma
12
12
12
24
12
Aluminum
Al
13
13
13
27
14
Sulfur
S
16
16
16
32
16
Chlorine
Cl
17
17
17
35
18
Isotopes (You should always use this word in the plural from) Isotopes are atoms of the same element which have a different number of neutrons and therefore also a different mass number. Since the number of protons is the same for all atoms; for the same element, the differences in mass must arise from different numbers and neutrons.
Example: Two common isotopes of chlorine are Cl-35 and Cl-37 (The numbers are their mass numbers);
75% of any chlorine sample contains Cl-35
and 25% of a chlorine sample contains Cl-37
There are more examples on isotopes;
Isotope
Atomic Number
Number of Protons
Number of Electrons
Mass Number
Number of Neutrons
Cl-35
17
17
17
35
18
Cl-37
17
17
17
37
20
H-1
1
1
1
1
0
H-2
1
1
1
2
1
H-3
1
1
1
3
2
C-12
6
6
6
12
6
C-14
6
6
6
14
8
a) What do all hydrogen isotopes have in common?
They all have 1 proton and 1 electron.
b) How are they different in terms of atomic properties?
The atomic mass.
c) How will you expect them to behave differently?
DIfferent boiling points.
Relative Atomic Mass, Ar
The relative atomic mass is the average mass of all isotopes of an element. It is calculated as shown below: Ar={(Relative isotopic mass #1x%)+(Relative isotopic mass #2x%)}/100
It is the relative atomic mass which is found in the Periodic Table
Electron Arrangement and the Periodic Table
Modern periodic table: Elements are in order of increasing atomic number
Group=Vertical column of elements
Period=Horizontal row of elements from an alkali metal to noble gas
The electrons in an atom orbit the nucleus in a series of electron shells (energy levels) which are at an increasing distance away from the nucleus. When working out the electrons are within an atom we need to follow a few simple rules;
The first shell (the lowest energy level) is always filled first, then the second then the third and so on; you start putting the electrons on the shells from the nucleus outwards i.e. fill up the first shell first then the second and so on.
Each electron shell can only hold a certain amount of electrons; in order of increasing distance from the nucleus, they are:
first shell: up to 2 electrons
second shell: up to 8 electrons
third shell: up to 8 electrons
and so fourth is all 8 electrons
The electronic structure of an atom tells us how many electrons there are in each shell. You should be able to draw the electronic structure of the first 20 elements in the periodic table.
So for example, K, Potassium has 17 electrons. They will be expressed as 2.8.8.1 numerically and will be drawn like the ones above.
Group number and number of electrons in outer shell are the same
The number of outer shell electrons or valency electrons determines the group number. The zero in Group refers to (in this case the column with He, Ne, Ar) the shell beyond the outer one, which does not have any electrons in it yet.
Period number and number of shells are the same
The number of the shell which is being filled determines the period to which an element belongs
Noble gas electron Configuration
Using the table above you will notice that all the elements in Group 0, the noble gases, have filled outer shells. This is a very stable electronic structure which makes these elements unreactive or inert.
HOME>SCIENCE>CHEMISTRY>1c: Atomic Structure
1c: ATOMIC STRUCTURE
1.8 Recall that atoms consist of a central nucleus, composed of protons and neutrons, surrounded by electrons, orbiting in shells1.9 Recall the relative mass and relative charge of a proton, neutron and electron
1.10 Understand the terms atomic number, mass number, isotopes and relative atomic mass
1.11 Calculate the ralative atomic mass of an element from the relative abundances of its isotopes
1.12 Understand that the Periodic Table is an arrangement of elements in order of atomic number
1.13 Deduce the electronic configurations of the first twenty elements from their positions in the Periodic Table
1.14 Deduce the number of outer electrons in a main group element from its position in the Periodic Table
Subatomic Particles
The nucleus of each atom specie can be described by just two numbers:
- The atomic number, Z, which is the number of protons in the nucleus of an atom;
- The mass number, A, which is the number of protons and neutrons in the nucleus of an atom
All this information about each atom can easily be shown in a short hand notation like in the example below which shows the short hand notation of a Lithium atom;The proton number is also equal to the charge on the nucleus and, as an atom is a neutral particle, to the number of electrons orbiting around the nucleus.
Using the short hand notation or your periodic table you should be able to find the number of protons, neutrons and electrons in each atom. Remember that in each atom:
Isotopes are atoms of the same element which have a different number of neutrons and therefore also a different mass number. Since the number of protons is the same for all atoms; for the same element, the differences in mass must arise from different numbers and neutrons.
Example: Two common isotopes of chlorine are Cl-35 and Cl-37 (The numbers are their mass numbers);
- 75% of any chlorine sample contains Cl-35
- and 25% of a chlorine sample contains Cl-37
There are more examples on isotopes;They all have 1 proton and 1 electron.
b) How are they different in terms of atomic properties?
The atomic mass.
c) How will you expect them to behave differently?
DIfferent boiling points.
Relative Atomic Mass, Ar
The relative atomic mass is the average mass of all isotopes of an element. It is calculated as shown below:
Ar={(Relative isotopic mass #1x%)+(Relative isotopic mass #2x%)}/100
It is the relative atomic mass which is found in the Periodic Table
Electron Arrangement and the Periodic Table
- Modern periodic table: Elements are in order of increasing atomic number
- Group=Vertical column of elements
- Period=Horizontal row of elements from an alkali metal to noble gas
The electrons in an atom orbit the nucleus in a series of electron shells (energy levels) which are at an increasing distance away from the nucleus. When working out the electrons are within an atom we need to follow a few simple rules;- The first shell (the lowest energy level) is always filled first, then the second then the third and so on; you start putting the electrons on the shells from the nucleus outwards i.e. fill up the first shell first then the second and so on.
- Each electron shell can only hold a certain amount of electrons; in order of increasing distance from the nucleus, they are:
- first shell: up to 2 electrons
- second shell: up to 8 electrons
- third shell: up to 8 electrons
- and so fourth is all 8 electrons
The electronic structure of an atom tells us how many electrons there are in each shell. You should be able to draw the electronic structure of the first 20 elements in the periodic table.So for example, K, Potassium has 17 electrons. They will be expressed as 2.8.8.1 numerically and will be drawn like the ones above.